1. Field of the Invention
This invention relates to improvements in the art of modular plastic turn belt conveyors.
2. Background and Prior Art
Conveyor systems in which a conveyor belt must alternatively travel in a straight line as well as making a lateral turn along its path are frequently used in various manufacturing or production operations. One major class of applications for such conveyor systems are commonly referred to as "turn belt conveyors." Such systems are generally utilized for conveying articles from one point in the process to another point while executing a lateral turn to avoid a building structural element, accommodate different orientations of systems inputting to and discharging from the conveyor, or minimizing conveyor transfer points.
Another major class of applications for conveyor belts which must travel in a straight line as well as making a lateral turn along their paths is commonly referred to as "spiral cage conveyor systems." Spiral cage conveyor systems, in which conveyor belts are driven in a spiral path with the belt curving edgewise around a series of vertically spaced loops, are commonly used to provide a great length of endless conveyor belt in a relatively small space. A particular advantage of such systems is that they can be used with high production volume operations while at the same time providing the long process dwell time required for certain processes, such as product warming or freezing.
One example of a spiral cage conveyor system is disclosed in U.S. Pat. No. 3,348,659, granted Oct. 24, 1967. In this system, the belt is frictionally driven. Tension is induced in the spiral belt so that there is frictional contact between the radial inner edge of the belt and a plurality of driving elements attached to a drive drum. This belt system is then frictionally driven by driving elements which slidingly engage the radial inner edge of the spiral belt loops. The driving elements move faster than the belt and continuously slide past the belt edge to achieve a frictional drive.
A well known means of constructing a conveyor belt which is capable of both running in a straight path and making lateral turns consists of incorporating a means whereby the belt pitch can collapse on the inner radius edge, permitting it to conform to the radius of curvature of the required turn. In such belts, the total tension in the belt, which results from the frictional and drive forces, is transferred to the outer radius edge of the belt. This creates two problems. First, since very large tensile forces must be borne by the outermost edge of the belt, the load carrying ability of the belt is limited by the strength inherent in the outer edge. Several techniques have been used to reinforce the outermost edges of turn belts in order to raise the load-carrying capacity to some usable value. One such technique is disclosed in U.S. Pat. No 3,439,795, granted Apr. 22, 1969 where reinforcing bar elements are placed on the outside edges of a flat wire conveyor belt.
A similar solution to this problem is also shown in U.S. Pat. No. 4,742,907, granted May 10, 1988, where reinforcing bar elements are placed on the outer edges of modular plastic conveyor belting. While being effective in increasing the load-carrying capacity of turn belts, these constructions still have certain limitations. One significant limitation occurs at the contact interface between the reinforcing link and connecting pivot pin. Since the entire belt load must be borne at this location, pivot pin and reinforcing link wear become excessive. Also, high shear stresses are present in the pivot pin because of the limited number of shear planes carrying the tensile load. Furthermore, the reinforcing links frequently fail due to the high fatigue loading they are subjected to.
There is a need in the art for an improved method for distributing the tensile load on link elements and shear load on pivot pins on the outermost edge of a modular plastic turn belt while traversing a turn.
The second problem occurs because the tension resident in the outer belt edge tends to cause the outside edge of the belt to "flip-up" vertically, upsetting the product, and causing damage to the belt and/or conveyor system.
To more clearly understand this phenomenon, one could visualize a turn belt where the outside edge consists of a large rubber band which has been tightly stretched. Such a stretched elastic element always seeks a position which reduces its energy state to zero. In other words, it wants to seek its shortest length. The belt cannot move directly inward because of its lateral stiffness and the rigid conveyor structure. Moreover, the outside belt edge cannot move downward because of the belt support elements. The only position that the belt can assume to minimize the length of its outer chord is to rotate or "flip" up about the inside belt edge. The only forces which counteract this tendency are the weight of the belt and weight of the product resting on the belt. For most metal conveyor belt designs, the weight of the belt itself is sufficient to prevent the belt from rising up under all but the most extreme conditions. However, the specific weight of plastic conveyor belting is generally much less than a similar metal belt. Accordingly, the tendency for a conveyor belt to rise up in the turn is greater with lightweight plastic conveyor belting. Several constructions have been employed in the past to prevent the outside edge of the belt from "flipping up."
One common arrangement is to put a rail above the outside belt edge to hold the belt down. This construction can be objectionable because it reduces the effective belt width and can result in conveyor projections in the product path. Another way of attacking this problem is disclosed in above-mentioned U.S Pat. No. 4,742,907 wherein "L" shaped projections on the underside of the belt cooperate with a similar projection on the conveyor structure. Such a design results in an overall thicker conveyor belt and hence a deeper, more costly conveyor structure. Additionally, the projections on the bottom of the belt prevent the bottom surface from being used as the product surface. This is often desirable in order to extend the life of the belt by alternating wear surfaces.
There is a need in the art for an improved construction and arrangement for retaining the outer edge of a modular plastic turn belt.
Another problem which commonly occurs with modular plastic conveyor belting is providing a means to reliably retain the plastic pivot rod within the belt.
Failure to accomplish such results in numerous problems, not the least of which is the belt actually falling apart in use. Other problems include interference between partially exposed rods and the surrounding conveyor structure. Additionally, as a practical matter the rods must be easily insertable and removable from the belt, as such is normally required during belt assembly, belt installation or belt repair. Furthermore, it is desirable to accomplish this without the use of any special equipment or tool. Such is particularly important when considering field installation and repair since special tools represent both added costs and inconvenience to the user.
Because of the significant problems that loose conveyor pins have caused, numerous methods have been used to capture the pivot rods connecting the links in plastic conveyor belts. Such methods include forming heads on the ends of the rods, but these heads can be knocked off and they must be removed for replacement of the rods. The heads have been provided by melting the ends of the rod to provide enlarged ends or heads which are larger in diameter than the rod hole and thereby prevent the rod from moving inwardly through the belt, i.e., the enlarged heads provide means to capture the rods. However, there are numerous problems with this solution to the problem of capturing the rods. First, special equipment is normally required to thermally form the heads. Secondly, the heads are exposed on the edges of the belt in a vulnerable location since any protuberance on a conveyor can either wear or knock the heads off the rods thus allowing the rods to fall out of the belt. Thirdly, there is a problem of the Poisson effect, i.e., when a material undergoes a change in dimension due to an elastic deformation along one axis, an opposite change in dimension or deformation occurs along a perpendicular axis. The amount of this opposite deformation is determined by Poisson's ratio. When the conveyor belt is in operation the rods are subject to compressive forces perpendicular to the axis of the rod. These compressive forces can deform the rod making the diameter of the rod smaller in accordance with the theory of elasticity. In accordance with the Poisson effect the rod then elongates along its axis; in effect, the rod becomes longer than its original length. This in turn causes the rod to protrude further beyond the edge of the belt causing further problems of interference with conveyor structure which can result in significant belt damage and possible down time.
Another way of capturing the rod within the belt is to form a circumferential bead the internal diameter of which is less than the diameter of the rod, the beads being formed at the ends of the rod holes. Such is shown in U.S. Pat. No. 2,911,091, granted Nov. 3, 1959. However such capturing of the rod is more or less permanent which doesn't take into consideration the need for disassembly and repair of the belt from time to time. Another solution to the problem of capturing a rod end is disclosed in U.S. Pat. No. 3,726,569, granted Apr. 10, 1973, in which the end of the rod hole and the outermost link end are plugged to prevent the rod from escaping from the belt. See also, U.S Pat. No. 4,709,807, granted Dec. 1, 1987. However, such plugs can be inadequate due to the rod elongation force caused by Poisson's effect mentioned above, and threaded plugs can cause stress risers and possible failure, in addition to extra manufacturing time and the cost of threading both the plug and the hole.
Another known method of capturing the rod is a snap-fitting end cap installed axially into the module rod hole or transversely into the module, blocking off the rod hole. However, the general design requirement for snap-fit assembly as currently known requires that the plug or end cap be flexible so that its snap projection can deform during installation. This flexibility, which is normally accomplished by placing the snap fit projection at the ends of two flexible arms, also weakens the plug or cap and reduces its ability to resist rod elongation forces. Further, end caps which are installed axially into the rod hole place the entire rod elongation force caused by the Poisson effect on relatively small snap-fit projections. This results in the rods "popping" the end caps off of the end modules.
It is also known to block the end of the pivot hole by insertable clips, for example, see, U.S. Pat. No. 4,893,710, granted Jan. 16, 1990 and U.S. Pat. No. 5,000,312, granted Mar. 19, 1991.
Despite all the prior work in this field, there is still a need in the art for improved arrangements for capturing and holding the rods interlinking the modules of modular plastic conveyors.
Another problem with modular plastic turn belts is the execution of a design which accommodates a small collapsing radius, with a rugged structural design with high tensile strength and high stiffness in resistance to gravity loads, yet at the same time being light-weight for economy, and having a high degree of open area for improved flow-through and heat transfer characteristics, and a generous clearance in the collapsed condition to accommodate foreign material build-up. An open type design which facilitates cleanability, and hence suitability for food contact applications, and which incorporates a positive sprocket drive which utilizes a broad sprocket tooth and resulting pitch line drive which is bi-directional and which can be accomplished from both the top surface and bottom surface of the belt is also difficult to execute while incorporating the other desired features mentioned above.
Two prior art modular plastic turn belts, aforementioned U.S Pat. No. 4,742,907 and U.S. Pat. No. 4,934,517, granted Jun. 19, 1990, will be discussed, vis-a-vis the desired design parameter mentioned above.
The belt of the U.S. Pat. No. 4,742,907 possesses a high percentage of open area and pitch line drive. However, due to the limitations imposed by attempting to achieve a high degree of collapsibility and the section thicknesses required in the design of a modular plastic turn belt to achieve required functional resistance to tensile loads, such belt design contains several drawbacks. First, because of the single elongated body, modules do not have significant stiffness in resistance to bending forces, nor does a belt offer adequate stiffness in resisting the forces imposed by gravity (product) loads. Moreover, the extended pitch of the belt of this patent is controlled at the outermost edge of the belt. It can be shown that by moving the extended pitch controlling position to a position inside the outermost belt edge, the collapsing radius of the belt can be decreased.
The belt of U.S. Pat. No. 4,934,517 includes a box-type structure for improved stiffness and link end clearance for improved collapsibility. However, this same box structure results in lower open area, increased difficulty in cleaning due to opposite facing internal surfaces, and the inability to execute pitch line sprocket drive.
There is a need in the art for a modular plastic turn belt system which offers an improved combination of strength and stiffness, open area, collapsibility, and pitch line drive.